5 sharps!
the f major scale only has b flat in it. the order of flats is BEADGCF and the last flat in the order becomes the key when the key signiture has one flat.
B Flat Concert
There are two flats in the key of B flat major: B flat and E flat.
B flat major, with keys B flat and E flat
B-flat and E-flat are found in B-flat major and G minor.
The key signature of the F flat major key has six flats: B flat, E flat, A flat, D flat, G flat, and C flat.
The subdominant in a scale is the 4th, which in the key of B-flat major is E-flat.
The key signature that has B flat, A flat, E flat and D flat is Concert A flat Major.
the scale e flat major has a flat, b flat and e flat.
F major: F G A Bb C D E F B is flat
Assuming the key signature has only one flat (B Flat) then the key is either F Major, or D minor.
B flat, E flat, A flat, D flat